Method and apparatus for a constant steam generation from the waste heat of an alkane dehydrogenation

ABSTRACT

The invention relates to a process and a device for an as constant as possible supply of steam flow from an alkane dehydrogenation, the process being carried out by passing a hydrocarbonaceous gas through reaction tubes loadable with a catalyst, and the reaction tubes, which are sealed towards the outside, running through a heating chamber which is heatable by means of burners, and the catalyst being regenerated for the reaction in a cyclic operating mode, in which the reaction is endothermic and the regeneration of the catalyst non-endothermic, and the capacity of the main burners is reduced during the regeneration of the catalyst, in which auxiliary burners, which serve to keep up the production of hot flue gas during the regeneration of the catalyst, are installed at the inlet of the flue gas duct to produce additional hot flue gas which is used for generating an as constant as possible amount of steam from the waste heat of the process.

The invention relates to one or more burners used as additional heating in the flue gas duct of a reactor for the performance of an endothermic reaction by which it is possible to produce a nearly constant amount of flue gas, the reactor being equipped with a steam generator located in the outlet of the flue gas duct of the heating chamber, and the burners being used as auxiliary burners for balancing the flue gas reduction in the flue gas duct of the heating chamber which normally occurs during the regeneration phase of the exothermic regeneration of the catalyst in use. The invention also refers to a device comprising one or more auxiliary burners installed in a reactor for the performance of an endothermic reaction including the necessary equipment by which it is possible to control the amount of flue gas in the outlet of the flue gas duct housing the steam generator. In this way, the steam flow obtainable from the steam generator is equalised to a substantial degree, which is of advantage in the operation of turbines or compressors. This device is especially suited for reactors typically used for alkane dehydrogenation processes.

A common process for performing an alkane dehydrogenation is to pass an alkane-containing hydrocarbon mixture across a dehydrogenation catalyst which causes the alkane contained in the gas mixture to react to the corresponding alkene. The catalyst is provided in a typical configuration in downward reaction tubes which the reaction gas enters through an inlet duct so that the product gas which contains, as a component, the required alkene can be obtained at the outlet of the reaction tubes. The reaction is endothermic so that it is required to heat the reaction tubes from the outside. This is normally accomplished by means of a reactor with a heating chamber with integrated reaction tubes, which can be heated with fuel gas. The reaction tubes are sealed towards the heating chamber. The heating chamber ends in a flue gas duct where the hot flue gas is thermally utilised and finally discharged into a stack.

A typical embodiment of the alkane hydrogenation process with an appurtenant device is described in WO 2004/039920 A2. A selection of various dehydrogenation processes and the corresponding catalysts involved is given in the publication by F. Buonomo, D. Sonfillipo, F. Trifirò, Handbook of Heterogeneous Catalysis, 1^(st) Edition, VCH, Weinheim, 1997 p. 2140 ff. and the literature references made therein.

The flue gas from the heating is discharged from the heating chamber via flue gas ducts. Its temperature is around 1000° C. depending on the respective constructional design. To further utilise the heat from the heating of the reaction tubes, a steam generator is typically installed for the flue gas in or downstream of the outlet of the flue gas duct.

An alkane dehydrogenation is typically accompanied by the formation of carbonaceous by-products which deposit on the catalyst after a certain reaction period. This will reduce the reaction yield and the production of required alkene. The reaction is, for this reason, interrupted after a certain period of time and the reaction gas flow across the catalyst stopped. In a typical embodiment an oxygen-containing gas is subsequently passed across the catalyst. It serves to remove the carbonaceous deposits and to regenerate the catalyst. After the regeneration, the alkane dehydrogenation in the respective reaction tube or reactor is resumed. According to this procedure the process is carried out in a cyclic operating mode.

As the dehydrogenation of alkanes is endothermic and the regeneration of the catalyst exothermic, the reactor requires a significantly lower amount of heat during the regeneration period than during normal operation. For this purpose, the burners are normally operated with less fuel gas during the regeneration phase resulting in an analogously lower production of flue gas.

WO 2007/118825 A1 describes a process for the production of olefins from hydrocarbons and a device for running the process. By switching off the burners during the regeneration phase the heat supply into the catalyst bed is interrupted so that no more heat is fed to the catalyst bed when oxygen-containing gas is passed across it during regeneration and the catalyst is prevented from overheating and degradation. For running the process the burners are provided with a switch-off device and are re-ignited for re-start by means of pilot burners after the regeneration. Both the heating burners and the pilot burners may be equipped with a monitoring device. No indications are made with reference to the generation of steam by the use of a steam generator and a compensation of the interrupted heating.

EP 179322 B1 describes a process for the exothermic regeneration of a catalyst which has been deactivated during an endothermic catalytic conversion in the course of a discontinuous process. By reducing the burner capacity to below 50 percent of the original capacity, preferably below 10 percent of the original burner capacity, which is accomplished by reducing the supply of heating fuel, savings in heating medium and combustion air can be achieved. As suitable application processes, special reference is made to the dehydrogenation of i-butane, n-butane or mixtures thereof. If several reactors are used, it is possible to operate them alternately so that there is in total no change in the supply of heating media, combustion streams and no load change in the waste heat system. This teaching as well does not give any indications with reference to the generation of steam by the use of a steam generator or to the compensation of the interrupted heating.

When the heating process output is decreased for the regeneration of the catalyst, the flue gas feed to the steam generators installed in the flue gas duct is reduced. This is problematic as in an important embodiment the steam generated by the steam generators is used for driving a compressor by means of a steam turbine. The dehydrogenation process works such that steam is supplied to serve as turbine steam during normal operation and also during regeneration. During the regeneration phase the unit itself produces less steam. At the same time, however, the steam consumption level in the regeneration mode is almost as high as during normal operation. The amount of steam supplied during the regeneration phase therefore determines the amount of steam supplied to the dehydrogenation unit.

It is therefore the aim to provide a process for the production of alkene by endothermic catalytic reaction making available as high an amount of flue gas as possible for the operation of a steam generator during the regeneration phase so that a constant amount of steam is provided throughout the whole “Production-Regeneration” cycle. It is another aim of the invention to provide a device for this purpose. The latter is also expected to allow monitoring and remote control of the process.

The invention achieves this aim by a process for the constant or controlled supply of flue gas from an endothermic catalytic reaction, by which it is possible to use this flue gas to produce a steam flow which is as high as possible in quantity by means of a steam generator, in which at least one auxiliary burner is installed in the outlet of the flue gas duct, the burner being used to generate a flue gas stream which does not get into contact with the reaction tubes to be heated and by which the flue gas stream is increased in quantity on the heat exchange surfaces of the steam generator during the regeneration. The invention achieves this aim also by a device made up by one or more auxiliary burners at the inlet of the flue gas duct of a reactor for the performance of an endothermic catalytic reaction, the auxiliary burners being arranged in the flue gas stream behind the reaction tubes. The device also includes a monitoring and control device for the auxiliary burners.

By the present invention it is possible to reduce the quantity of steam supplied. In this way, no additional costs are incurred by heat-exchange devices as equipment and internals dimensioned for normal operation are used. The auxiliary burners, which are mainly required for the regeneration mode, are comparatively inexpensive.

Typical endothermic catalytic processes suitable for the application of the invention are alkane dehydrogenation processes. These are, in any case, reactions which are performed in reaction tubes loadable with catalyst, the reaction tubes being arranged in a heatable reaction chamber and the reaction chamber being heated with fuel gas from burners. A typical process for the dehydrogenation of alkanes suited for the application of the invention is described in WO 2004/039920 A2. This document also describes a reactor in which the hydrogen generated in the alkane dehydrogenation is incinerated in a separate process step.

Especially claimed is a process for the regeneration of a fixed-bed catalyst with temporally constant generation of steam from the heating of the reactor, in which

-   -   a fixed-bed catalyst is arranged in one or several reaction         tubes where an endothermic reaction is carried out by passing         through a reaction gas mixture, and     -   the reaction tube or tubes are heated from the outside by         combustion of a fuel gas in a heating chamber housing the         reaction tubes for carrying out the endothermic reaction, and     -   the reaction in the reaction tube or tubes is performed in a         cyclic operating mode over a limited period of time, the period         not used for the reaction being used for the regeneration of the         catalyst by passing an oxygen-containing or water         vapour-containing gas or a mixture of both across the catalyst,         and     -   the flue gas stream produced by the heating of the reaction         tubes is discharged from the heating chamber and utilised for         steam generation by a steam generator,         and characterised in that     -   at least one auxiliary burner is installed in the outlet of the         flue gas duct, this burner producing a flue gas stream not         getting into contact with the reaction tubes to be heated and         increasing the flue gas stream in quantity on the heat exchange         surfaces of the steam generator during the regeneration.

By the increased amount of flue gas in the flue gas duct it is possible to use the heat exchange surfaces in the flue gas duct during the regeneration period of the catalyst more efficiently. In this way, the amount of produced steam can be kept nearly constant throughout the whole duration of the process. For this purpose, the auxiliary burners are equipped with a control device by which it is possible to control the amount of combustion gas. This can be achieved by controlling the auxiliary burners by means of the flue gas temperature in the flue gas stream downstream of the auxiliary burners. The control device adjusts the supply of fuel gas or combustion air into the auxiliary burners.

To further increase the steam amount generated during the regeneration, the auxiliary burners are used advantageously such that the temperature of the flue gas stream at the inlet of the flue gas duct is increased on the heat exchange surfaces of the steam generator. The temperature in the flue gas duct and on the heat exchange surfaces can be controlled via the fresh air supply into the flue gas duct if this is required.

The auxiliary burners are advantageously equipped with a control device so as to control the fuel gas supply and thus the production of flue gas. The control device is governed by a temperature sensor provided near the heat exchange surfaces of the steam generator so that it is possible to control the auxiliary burners by means of the temperature in the flue gas duct. In a simpler embodiment it is also possible to provide for a manual control of the auxiliary burners.

The control device of the auxiliary burners can also be controlled by the amount of produced steam. In such case, a steam flow meter for the amount of steam produced is installed in a suitable position of the steam generator so that the auxiliary burners can be controlled by the amount of steam produced.

Processes which are qualified for the process according to the invention are in particular alkane dehydrogenation processes used to convert an alkane into an alkene by releasing hydrogen. This may be carried out in a single process. It is, however, also common practice to carry out the alkane dehydrogenation converting an alkane into an alkene by releasing hydrogen and oxidising the hydrogen in a subsequent separate process step in which a further dehydrogenation of not yet converted alkane is achieved. The auxiliary burners may then be installed in one or more reactors. In this way the whole endothermic process is supported by the regeneration of the catalyst.

Also claimed is a device by which it is possible to run the process according to the invention. Especially claimed is a device for the generation of a constant amount of steam from the waste heat of an alkane dehydrogenation process, comprising

-   -   a reactor with heating chamber for carrying out an endothermic         reaction with integrated reaction tubes which can be loaded with         a catalyst, an inlet for the reaction gas and an outlet for the         product gas in the reaction tubes, one or more main burners         which do not get into contact with the catalyst or the reaction         gas and heat the reaction tubes in the heating chamber from the         outside, and a flue gas duct with outlet for the flue gas at the         end of the heating chamber, and one or more steam generator/s in         the flue gas duct,         and characterised in that     -   one or more auxiliary burners are installed at the outlet of the         flue gas duct behind the reaction tubes and upstream of the         entry into the steam generator or generators.

Reactors for the performance of endothermic processes which are equipped with auxiliary burners for bridging the start-up procedure are known according to the state of the art. US 2003/0101651 A1 describes a device for an endothermic catalytic reaction by which a hydrocarbonaceous gas is passed through tubes which can be loaded with catalyst and are heated from the outside, the reaction gas being heated by convection in counter-current flow. By the reaction arrangement it is possible to significantly reduce the size of the reactor and to design the complete assembly as a mobile unit. The device describes auxiliary burners used to start the reaction by installing them in the heating chamber for combustion of the fuel gas. Not described is a regeneration of the catalyst or interruption of the heating process. Neither mentioned is a control device for the auxiliary burners.

In most embodiments of the invention the auxiliary burners are provided with control devices by which it is possible to control the capacity of the burners. Suitable control devices are, for instance, valves, gate valves, flaps or stems which serve to control the supply of fuel gas into the auxiliary burners. The control device may also be used to control the supply of combustion air into the auxiliary burners.

The auxiliary burners may also be controlled by parameters measured in the outlet of the flue gas duct. Devices required according to the related process claims are especially sensors measuring the flow rate of the combustion gas or the temperature of the combustion gas. To serve this purpose, these are installed in the outlet of the flue gas duct. If the aim is to control the auxiliary burners by the combustion gas flow rate, the flue gas ducts are equipped with a measuring device which serves to measure the combustion gas flow rate of the flue gas in the flue gas duct, by which it is possible to control the auxiliary burner.

If the aim is to control the auxiliary burner/s by the temperature of the flue gas stream, the flue gas ducts are provided with a device for measuring the temperature of the flue gas in the flue gas duct by which it is possible to control the auxiliary burner. If a comparison measurement in comparison to the overall flow rate or temperature of the flue gas is required, appropriate sensors can be installed in the flue gas duct itself or on the heat exchange surfaces of the steam generator. In another embodiment it is also possible to use Lambda probes for measuring the oxygen content in the flue gas duct, should the auxiliary burners be controlled by the oxygen content in the flue gas duct.

The reactor for integrating the device according to the invention is typically configured as is common practice according to the state of the art. For carrying out the invention this includes a reactor for carrying out an endothermic reaction with integrated reaction tubes which can be loaded with a catalyst, burners which do not get into contact with the catalyst or the reaction gas and heat the reaction tubes from the outside, an inlet for the reaction gas and an outlet for the product gas in the reaction tubes, an inlet for the fuel gas and a flue gas duct, and a steam generator with heat exchange surfaces in or downstream of the outlet of the flue g as duct. The main burners and the auxiliary burners according to the invention may be installed in any position in the heating chamber or in the flue gas duct. This applies analogously to the heat exchange surfaces to be heated. The burners, reaction tubes or steam generators can be provided as single or as multiple units. The auxiliary burners are, in any case, arranged such that the escaping flue gases do not get into contact with the reaction tubes and the enclosed catalyst.

To control the auxiliary burners especially such devices may be used that serve the purpose of burner control according to the state of the art. These are typically valves, gate valves, flaps or stems which serve to control the supply of fuel gas or combustion air into the auxiliary burners. To measure control data, especially thermocouples, pressure gauges, gas flow meters and oxygen probes may be used.

Used as auxiliary burners may be gas burners, liquid-fuel burners, rocket burners or solid-fuel blower burners. The type is determined by the size of the flue gas duct and of the heat exchange surfaces. The auxiliary burner device according to the invention also includes suitable ignition devices as there are, for example, electric or electronic igniters, pilot burner or flint stones. The auxiliary burners are preferably equipped with a control device by which the capacity of the auxiliary burner/s can be controlled. This can be implemented such that the flue gas duct, for example, is equipped with a device for measuring the flue gas temperature in the flue gas duct and by which the capacity of the auxiliary burner/s can be controlled.

To additionally utilise the waste heat, steam generators are used which may be arranged as desired and provided in any number desired. Typically these are steam generators which are arranged as indirect heat exchangers with heat exchange surfaces. These may be of optional design. These may also include measuring devices for measuring the amount of steam produced. The steam generators which are heated by means of the auxiliary burner/s may be equipped with a steam flow meter by which it is possible to control the capacity of the auxiliary burner/s. Furthermore included in the device according to the invention is ancillary equipment such as pipelines, for example. These may be of optional design and number.

The device according to the invention involves the advantage that an amount of steam which is as constant as possible throughout the duration of the process can be generated from the waste heat of an alkane dehydrogenation. By the device and the process according to the invention the generation of steam from the waste heat of the before-mentioned processes can be optimised and used to recover mechanical energy. 

1.-11. (canceled)
 12. A process for the generation of a constant amount of steam for the waste heat of an alkane dehydrogenation process, comprising: arranging a fixed-bed catalyst in one or more reaction tubes and carrying out therein an endothermic reaction by passing a reaction gas mixture through the reaction tube(s); heating the reaction tube(s) from the outside by combustion of a fuel gas in a heating chamber housing the reaction tubes, forming a flue gas stream; performing the reaction in the reaction tube(s) in a temporally cyclic operating mode, a period of time not used for the reaction being used for the regeneration of the catalyst by passing an oxygen-containing gas, water vapour-containing gas, or mixture thereof across the catalyst, and utilizing the flue gas stream for steam generation by a steam generator having a heat exchange surface; and increasing the quantity of the flue gas stream which contacts the heat exchange surface of the steam generator during regeneration by activating at least one auxiliary burner installed in the flue gas stream behind the reaction tubes to be heated, this burner producing a flue gas stream which does not contact the reaction tubes.
 13. The process of claim 12, wherein the auxiliary burner(s) are used to increase the temperature of the flue gas stream on the heat exchange surfaces of the steam generator during the regeneration.
 14. The process of claim 12, wherein the auxiliary burners are equipped with a control device and the flue gas duct which is heated by means of the auxiliary burner is provided with a temperature measuring device to control the auxiliary burners by means of the temperature in the flue gas duct.
 15. The process of claim 12, wherein the auxiliary burners are equipped with a control device and the steam generators which are heated by the auxiliary burner are equipped with a steam flow meter to control the auxiliary burners by the amount of produced steam.
 16. The process of claim 12, wherein the endothermic process with the catalyst to be regenerated is an alkane dehydrogenation process in which an alkane is converted into an alkene by the release of hydrogen.
 17. The process of claim 12, wherein the endothermic process with the catalyst to be regenerated is an alkane dehydrogenation process in which an alkane is converted into an alkene by the release of hydrogen and the hydrogen is oxidised in a subsequent process step in which a further dehydrogenation of not yet converted alkane is achieved.
 18. A facility for the generation of a constant amount of steam from the waste heat of an alkane dehydrogenation process, comprising: a reactor with heating chamber for carrying out an endothermic reaction with integrated reaction tubes which are loaded with a catalyst; an inlet for a reaction gas and an outlet for a product gas in the reaction tubes; one or more main burners which do not contact the catalyst or the reaction gas and which heat the reaction tubes in the heating chamber from the outside; a flue gas duct at the end of the heating chamber and one or more steam generator(s) in the flue gas duct; and one or more auxiliary burner(s) installed in the inlet of the flue gas duct in the flue gas stream behind the reaction tubes and upstream of the entry into the steam generator or generators.
 19. The facility of claim 18, wherein the auxiliary burner(s) are equipped with an ignition device.
 20. The facility of claim 18, wherein the auxiliary burner(s) are equipped with a control device which serves to control the capacity of the auxiliary burner(s).
 21. The facility of claim 18, wherein the flue gas duct is equipped with a device for measuring the flue gas temperature in the flue gas duct, by which the capacity of the auxiliary burner(s) is controlled.
 22. The facility of claim 18, wherein the steam generators which are heated by the auxiliary burner(s) are equipped with a steam flow meter to control the capacity of the auxiliary burner(s). 